It is well-known that a RAM accelerator is an aero-thermo-chemical device comprising a powder or light-gas preaccelerator which can set the projectile to be fired in motion up to a supersonic velocity. Several tubes are arranged in series in the extension of the preaccelerator which are separated from each other by plastic diaphragms and filled with a reactive gas mixture. This gas mixture is activated during the passage of the projectile so that an additional thrust acts on the projectile.
The main advantage of a RAM accelerator lies in its capacity to impart a high initial velocity at a low acceleration. This is due to the fact that the energy is distributed along the RAM stages instead of being concentrated in the combustion chamber as is general practice in conventional accelerators.
A projectile for RAM accelerators is normally composed of at least two main parts: the projectile itself which shall be accelerated to the desired velocity and its sabot.
The external form of the projectile is designed to the classical aerodynamic rules so as to provide, on the one hand, the equivalent of a well-dimensioned diffuser in the volume between the projectile and the wall of the acceleration tube and, on the other hand, the desired type of flow (e.g. a normal shock wave acting on the boattail of the projectile in the case of a subsonic combustion).
The sabot which has the same diameter as the tube shall satisfy the following conditions to be effective:
In the presence of the projectile to be accelerated it shall withstand the maximum acceleration in the preaccelerator.
It shall contain one or several sealing elements so that the thrust furnished by the preaccelerator can be transmitted without any losses.
As the risk of deterioration of the tube has to be taken into account it shall offer a sufficient guiding length (40% to 60% of the tube's caliber) so that it will neither rotate during its installation nor at the moment the thrust is achieved.
It shall ensure the initiation of the diffuser during the free flow between projectile and tube wall.
The U.S. Pat. No. 4,982,647 describes a sabot which is perforated on both sides by a multitude of orifices with axes parallel to the direction of flow and offering a total flow passage cross section proportional to the free section at the projectile base. During the preacceleration phase the rear of the sabot is closed by a pressure plate or a valve. As soon as the medium in front of the sabot has established a sufficient back pressure the valve is automatically opened, and thus the diffuser is initiated. The tube is no longer plugged. For big calibers and relatively high pressures, this solution presents some problems with the mechanical strength of the sabot, because the latter loses its mechanical strength once it is perforated. The sabot is therefore reinforced in advance, and depending on the material used this may lead to excessive length and additional weight. These unfavourable characteristics may involve a supplementary handicap in the case of subsonic combustion: With regard to the initiation delay they may impede the establishment of an adequate volume between projectile and sabot at the moment of ignition where the combustion can take place. In certain cases the sabot can be solid.
Thus the main reasons for the disadvantages of the known sabots are as follows: their relatively great axial dimension which exceeds 40% of the caliber in the above-mentioned patent, their diameter which is equal to the caliber, and their relatively high weight which may exceed 20% of the projectile weight. All these aspects affect the ballistic performances of the system in an unfavourable way.